Peptide helices serve as models for helical regions in proteins. As such, they can be used to develop insight into the protein folding problem, probe how mutations affect helical structures, examine factors that stabilize helices and explore conformational changes similar to those involved in protein function, Recent discoveries using spin label Electron Spin Resonance (ESR) suggest that peptide helices coexist in two conformations: 3(10)-helix and alpha-helix. The former structure possesses i->i+3 hydrogen bonding and the latter i->i+4 hydrogen bonding. The relative populations of these two conformations are sensitive to pep tide length and amino acid composition. The 3(10)-helix -> alpha-helix equilibrium is involved in the function of numerous enzymes and 3(10)- helix is a common structural element in proteins. This proposal seeks to build upon these findings by using ESR to examine three facets of designed helix structure. First, refined spin label methods are proposed to quantify the relative populations of 3(10)- and alpha-helix. These methods employ rigid spin labels and mixed isotope experiments in order to develop an inter-label distance ruler. The second project focuses on the influence of modified amino acids on helix structure. In particular, the common posttranslational modification of phosphorylation will be explored. Phosphorylation triggers conformational transitions in proteins, yet the mechanism behind such triggering is not understood. The proposed studies on phospho-peptides will reveal mechanisms by which phosphorylation changes the helix propensity of her residues. The third project is a study of helices implicated in diseases. Recent research suggests that destabilization of specific protein helices is the first step in certain diseases. For example, prion infection is associated with a loss of helicity in the core region of the cellular prion protein PrPC. In another example, the tumor suppresser p53 protein, which is responsible for approximately 50% of all known human cancers, exhibits reduced function from mutations of the DNA binding helix. Peptide models will be developed to explore the local helix structural transitions involved in both of these proteins.